Large area imagers, CCDs and CMOS, are required to be flat to capture a quality image. For many applications, it is required that large area imagers be manufactured to form a substantially flat (with a deviation from flatness of less than 15 microns) active imaging surface over the entire active imaging area. A large area image sensor assembly is defined as a packaged imager having an active sensor area of 20 mm by 20 mm or larger. Currently, these CCD or CMOS imagers are composed of an imager die mounted on either a substrate or mounted in an electronic package. When an imager die is mounted on a substrate, there are several deficiencies that result. The die and bond wires are not protected from damage or debris. Additional potting of the wires or additional structural elements must be added to protect the die and wires. Without these additions, the imager remains unprotected. When currently available electronic cavity packages are used, the wires and imager are protected but the flatness of the imager is not sufficient to meet the needs of many applications including medical imaging sensors and large format digital cameras. Current electronic packages use high temperature methods to join the package components. These high temperatures approximately 400° C. and higher are used to either melt glass, braze or co-fire a ceramic package as methods to join components together. These high temperatures and fastening techniques cause the critical die attach area to which the imager is attached to the electronic package to warp. The die attach area needs to be flat to create a flat imager. Since these are cavity packages, they impede post grinding of the die attach area to repair the warping or bowing of the imager plane created during the high temperature fabrication processes.
Other methods of packaging an image sensor include mounting an imager into an injection molded thermoplastic resin package as disclosed by H. Yamanaka in U.S. Pat. No. 5,529,959. The assembly process includes injection molding of a base with an incorporated lead frame. This patent discloses that small imagers can be made flat by this process without quantifying a definition of flatness. Although injection molding is highly successful for the manufacturing of small imager packages, it is extremely difficult to achieve a base flatness of 10 μm or less in larger areas in excess of 25 mm by 25 mm or larger. U.S. Pat. No. 5,382,310 by Ozimek et al. describe a method to make small conventional solid state image sensors by directly bonding an imager die to a conductive lead frame. The imager bonding pads are then wire bonded to the lead frame. The lead frame and imager are then encapsulated top and bottom with adhesive to provide structural strength. This approach is not amenable to making large area flat imagers. U.S. Pat. No. 6,121,675 by Fukamura et al. describes a method which utilizes silicone to cover the die and wirebonds thus preventing moisture and dirt egress. The flexible material prevents the wire bonds from breaking.
Although the currently known and utilized methods for producing an image sensor assembly are satisfactory for many applications, they include drawbacks. The imagers produced by conventional packaging methods do not have sufficient flatness after the brazing or glass melting process to meet the requirements for large size medical imaging sensors and large format digital cameras. In addition, the flat substrates are not enclosed which obviously limits their ability to mount optical coverglass or protect the sensor and wire bonds.